This invention relates to pressure swing adsorption (PSA) processes, and more particularly to hydrogen production via pressure swing adsorption processes.
The process of production and recovery of hydrogen by steam and/or air reforming of hydrocarbon rich gas streams, such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons, is well known in the art. Typical commercial sources for the production of hydrogen include reforming of natural gas or partial oxidation of various hydrocarbons. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g., air or oxygen-enriched air), producing a hydrogen gas stream containing accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless it is desired to recover carbon monoxide, it is customarily converted to carbon dioxide by water gas shift reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then sent to a PSA unit. Other hydrogen-rich gas sources that can be upgraded by PSA technology to a high purity product include refinery off-gases with C.sub.1 -C.sub.6 hydrocarbon contaminants. See, e.g., U.S. Pat. No. 3,176,444 to Kiyonaga.
In PSA processes, a multi-component gas is passed to at least one of a plurality of adsorption beds at an elevated pressure to adsorb at least one strongly adsorbed component while at least one relatively weakly adsorbed component passes through. In the case of hydrogen production via pressure swing adsorption (H.sub.2 PSA), H.sub.2 is the weakly adsorbed component which passes through the bed. See, e.g., U.S. Pat. Nos. 3,430,418 to Wagner, 3,564,816 to Batta and 3,986,849 to Fuderer et al. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more steps which permits essentially pure H.sub.2 product to exit the bed. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization. H.sub.2 PSA vessels generally contain a mixture of activated carbon, for bulk CO.sub.2 and CH.sub.4 removal, followed by a molecular sieve for CO and N.sub.2 removal. See, e.g., U.S. Pat. No. 3,430,418 to Wagner.
H.sub.2 PSA is a multi-million dollar industry supplying high purity hydrogen for chemical producing industries, metal refining industries and other related industries. The cost of hydrogen from integrated reformer/PSA systems is impacted by both the capital and operating costs of the system. Clearly, economic production of hydrogen requires as low as possible operating and capital costs. Capital cost is largely dictated by the size of the reformer and the size of the PSA beds. PSA bed size decreases as the hydrogen productivity (lb-moles of hydrogen produced/bed volume) of the PSA increases. Hydrogen productivity can be increased by either improved process cycles or improved adsorbents. The size of the reformer is impacted mostly by the hydrogen recovery of the PSA. Improvements in hydrogen recovery in the PSA results in smaller reformer size (as there is a diminished need to produce hydrogen out of the reformer because of better recovery in the PSA). Improvements in hydrogen recovery also result in a reduced demand for reformer feed gas, i.e., natural gas, which generally constitutes the largest operating cost of the reformer. Hydrogen recovery in the PSA can also be improved by either improved process cycles or improved adsorbents.
H.sub.2 PSA process performance (on-line time, productivity, product purity) is largely dictated by the most weakly adsorbing component in the H.sub.2 -rich stream. A bed can stay on feed, producing pure H.sub.2, only until the level of impurity breakthrough reaches the desired product purity. For steam/methane reformer (SMR) cases, the PSA feed gas composition is typically about 1% N.sub.2, 5% CH.sub.4, 5% CO, 18% CO.sub.2 and the remainder H.sub.2. To produce high purity H.sub.2 (99.99+%) with this feed gas composition, N.sub.2 is the key breakthrough component since it is the most weakly adsorbing feed gas component. Since N.sub.2 is the key breakthrough component, it has been common practice to place a zeolite adsorbent with high capacity for N.sub.2 at the product end of the bed. See, e.g., U.S. Pat. No. 3,430,418 to Wagner, which teaches a layered adsorption zone with the inlet material comprising activated carbon and the discharge end containing zeolite for removing the minor component of N.sub.2, CO or CH.sub.4 ; U.S. Pat. No. 3,564,816 to Batta, which exemplifies only the use of CaA zeolite for H.sub.2 PSA processing; and U.S. Pat. No. 3,986,849 to Fuderer et al., which teaches a layered bed adsorption zone with activated carbon at the feed end of the bed and CaA zeolite at the discharge end.
The H.sub.2 -rich gas stream may also contain oxygen and argon impurities, particularly if partial oxidation of hydrocarbons is the route to hydrogen production. Since these impurities are more weakly adsorbing on zeolite adsorbent than N.sub.2, their breakthrough determines bed on-line times and product purity. However, there is very little prior art specifically teaching about Ar and/or O.sub.2 removal from the H.sub.2 -rich gas streams.
U.S. Pat. No. 3,430,418 to Wagner discloses Ar as a selectively adsorbable feed gas component and teaches a layered carbon/zeolite adsorption zone, failing to appreciate the surprising disadvantages of using a zeolite-based adsorbent for H.sub.2 PSA with gas streams containing Ar in addition to H.sub.2, CO, N.sub.2, etc. The layered adsorption zone comprises a first layer of activated carbon for water, methane and CO.sub.2 removal, followed by a zeolite layer for CO and N.sub.2 removal. Wagner also teaches an all zeolite adsorption zone for H.sub.2 PSA applications.
U.S. Pat. No. 3,564,816 to Batta also mentions Ar as a feed gas component and teaches a single adsorption zone of zeolite.
U.S. Pat. No. 3,176,444 to Kiyonaga describes appropriate adsorbents for given PSA separations. The removal of Ar and/or O.sub.2 from H.sub.2 is not addressed.
U.S. Pat. No. 4,077,780 to Doshi teaches a PSA process for the recovery of H.sub.2 and N.sub.2 from ammonia plant purge gas. The process comprises the use of activated carbon as the preferred adsorbent, and operation beyond the point of CH.sub.4 breakthrough in the course of the adsorbent bed void gas recovery steps, to allegedly reject substantial amounts of Ar and CH.sub.4 per unit amount of N.sub.2 and H.sub.2 produced. This process is unsuitable for producing high purity H.sub.2, as the product gas contains unacceptably high levels of impurities, such as N.sub.2 and Ar.
The inventors are not aware of any prior art patents which mention O.sub.2 removal from H.sub.2 -rich PSA feed streams.
Accordingly, there has been a need for improved H.sub.2 PSA processes comprising the use of preferred adsorbents to isolate high purity H.sub.2 from gas streams containing H.sub.2, CO, and O.sub.2 impurities and/or Ar impurities.
All references cited herein are incorporated herein by reference in their entireties.